Crust and Uppermost Mantle Structure of the Alaska Subduction Zone from Joint Inversion of Rayleigh Wave Dispersion and Receiver Functions

Abstract:

The Alaska subduction zone provides a unique opportunity to study the structure and hydration of the forearc and incoming plate in a region with strong along-strike gradients in seismogenic properties and incoming plate fabric. Here we image the crust and uppermost mantle structure of the Alaska subduction zone using the newly acquired data from the Alaska Amphibious Community Seismic Experiment (AACSE; May 2018 – September 2019) and existing broadband seismic stations on the Alaska Peninsula including the EarthScope Transportable Array and other Alaska regional networks. Altogether 61 AACSE ocean bottom seismographs (OBSs) and 118 land stations are utilized. We use ambient noise cross-correlation to retrieve the Rayleigh wave phase and group velocities at shorter periods (7-30 s) and Helmholtz tomography to determine Rayleigh wave phase velocities from teleseismic surface waves at longer periods (20-100 s). All the OBS data are pre-processed to remove the seafloor tilt and compliance noise. The local phase velocity dispersion curve at each node is obtained from ambient noise and earthquake measurements by weighted averaging in the overlapping period band. We also perform receiver function analysis for the land stations to better constrain the Moho depth beneath each site. A 3-D isotropic shear velocity model is finally constructed by a joint Bayesian Monte Carlo inversion of Rayleigh wave phase/group velocities and receiver functions. The results show a low shear velocity zone in the uppermost mantle of the incoming plate near the Shumagin Islands, where large incoming plate faulting is observed and previous active source studies find P-wave velocity reductions (Shillington et al., 2015). The velocity reduction is likely due to the serpentinization of the incoming plate mantle, but the depth extent and magnitude is not as strong as that is observed in the Mariana subduction zone (Cai et al., 2018), suggesting a more modest hydration of the Alaska slab.